Static resistance electric initiator



Aug. 13, 1957 c. F. HQRNE STATIC RESISTANT ELECTRIC INITIATOR AFIGB FIG? FiG

Filed Nov'. 9, 195s FIG.6

FIG. 2

FIG.

CHARLES F, HORNE INVENTOR.

A BY M 09M AGENT United States Patent titice 2,802,422 Patented Aug. 13, 1957 stanza sTArIc RESISTANCE Ernc'rnrc INITIAToR Charles F. Horne, Kingston, `N. Y., assgnor to Hercules Powder Company, Wilmington, Del., a corporation ci' Delaware Application November 9, 1953, Serial No. 391,097

13 Claims. (Cl. lim- 28) This invention relates to electric initiators and more particularly to electric initiators which are highly resistant to premature tiring by static electricity.

The art has long recognized the dangers inherent in accidental discharge of electric initiators by means of static electricity. Accidents, which at 'the timeof their occurrence have seemed without explanation, have been subsequently traced to the firing of an initiator by a static discharge. Since normally used ignition compositions necessarily are highly heat sensitive, a discharge of relatively high voltage is quite capable of igniting the composition and tiring the initiator. The art has generally considered that such accidentaltirings result from a direct discharge from a lead Wire to a metallic initiator shell in the locus of the ignition composition. Obviously, caps can also be tired by direct discharge .through the bridge wire via the two lead wires. What isnot so well understood, however, is the fact that an initiator can also be fired by the passage of a static voltage through the bridge wire when the discharge is from shunted lead wires to shell at a point other than through the explosives charge.

While the danger of premature ring due to static discharge is present in all types of electric initiators, the procedures employed in seismographic prospectingand the sensitive ignition compositions employed in seismic-type blasting caps have tended to make this type of initiator lmore susceptible to static than regular electric blasting caps and delay electric blasting caps.` Until very recent years, commercial seisrnograph caps ,generally .required a discharge in the order `of 5000 volts to `lire the cap by direct discharge through the ignition `composition and a discharge in the order of 12,000 volt to lire `the caps by heating of the bridge wire when the charge was supplied by a 750 micromicrofarad capacitor and the discharge was from shunted lead wires to shell. It has been established `that voltages of this `magnitude may be developed by a man under proper conditions and that much higher voltages may be generated in equipment employed by seismic prospectors such as drill rigs and trucks. Actually,.condi tions favoring high voltage static generation, such as low humidity, high winds, sandstorms, and the like, are common in the localities where most seismic prospecting is done. Consequently, it will be seen thata definite danger of accidental firing of seismic caps is present under such adverse conditions. As a result of this danger, seismo- `graphic prospecting organizations and blasting cap manufacturers have been constantly attempting to raise the static resistance of all electric initiators and especially that of the seismic-type blasting cap.

A blasting cap structure has previously been proposed in which a lead wire is disposed in contact or almost in contact with the shell, at a point removed from the ignition composition, in order to provide for a discharge of the static electricity from the lead wire to the shell. However, this type of structure offers good protection from static or tiring of the cap only when the charge passes down the one lead wire to the shell. Little or no benefit is obtained when the current passes through both lead wires. Even when both lead wires are so disposed, discharge will occur from only one Wire in :many instances and protection will be limited as far as heating of bridge is concerned.

A structure has `also been proposed wherein one or both of the bared lead wires are connected to the shell by means of semiconductive material outside the locus of the ignition composition. This construction affords a considerable improvement in static resistance. However, it is very dicult with such a structure to maintain a proper balance of conductivity `that wili allow a discharge from both wires to the shell and still have sucient resistance for protection against the low voltage currents which attend many commercial blasting operations. Additionally, it has been found that in some instances, the static resistance of this type of structure diminishes with storage. Furthermore, such caps have been known to tire when 10-40 volts from a battery are applied between lead wires and shell.

In another proposed structure, conductive material is disposed about the bared lead wires and extends to the shell. This conductive material acts as a true resistor in that the resistance is low, normally from 10-100 ohms. The resistance of such a body of material is similar to that of a regular carbon resistor, being fairly constant, but subject to variation due to temperature. The resistance does not change greatly due to passage of current until the current is suicient to cause heating. This type of .cap gives good static protection in most instances. However, it has been found that in some instances discharges occur from only one wire which allow a firing of the cap by the heating of the bridge wire. Even more than in the case of the semiconductive material, this structure has a serious deficiency of insulation from shell to lead wires and can be tired in this manner with a very low voltage. In other words, this structure, while removing a considerable part of the hazard of static electricity, has introduced an equally undesirable hazard in the form of undesirably low resistance between lead wires and shell.

In still another structure, it has been proposed to equip the bared lead Wires with protrusions which extend toward the shell. This structure is usually employed with a .matchhead ignition element which is insulated. However, a discharge usually takes place from only one wire and a tiring of the cap by the heating of the bridge wire is thus permitted. ln addition, a hotter spark is obtained when the discharge is directed from localized points. Even though the protrusions are outside the locus of the ignition composition, such violent discharges are to be avoided when the same results can be obtained in a less violent manner.

While all `of these preceding structures give an initiator a measure of static resistance, it will be `seen that 4in each case, the protection from static is not complete and, in most instances, what protection is obtained is brought by a structure which is characterized by low voltage breakdown. Furthermore, these structures are not characterized by a suliicient resistance to heating ofthe bridge wire by a static discharge through the bridge Wire itself.

In copending application Serial No. 340,821, tiled March 6, 1953 (now U. S. `Patent No. 2,658,451, issued November l0, 1953) there is disclosed a structure which gives greatly improved performance in that low static susceptibility and high resistance to tiring by stray currents are simultaneously achieved. lProtection .fromstatic discharge is obtained by disposing a semiconductive body lit may be a sleeve, a ferrule, or a shell liner.

semiconductive body and jumps to the grounded shell out- The present application is a continuation-in-part of application Serial No. 340,821, now U. S. Patent No. 2,658,451 and is directed to a static resistant initiator which is of improved design and performance and is easier to manufacture.

I Generally described, the present invention is a static resistant electric initiator having in combination a metal- ',lic shell, a pair of lead wires, an ignition plug of semiconductive material disposed about bared portions ofthe lead wires within the shell, a bridge wire joining the terminals of the lead wires below the'lower surface ofthe plug, an ignition composition disposed about the bridge wire,` a spacing body of dielectric material disposed about the lowe'r portion of the plug between the plug and shell wall, the portion of the plug above thespacing body ap- 'proachingthe'shell wall but being separated therefrom by a distancesubstantially less than that between either lead wire and the Vshell in the locus of the ignition composition, and the total resistance between the shell wall and plug being substantially less above the spacing body than at any other `point on the plug. As will be hereinafter described, the semiconductive ignition. plug may be of 'uniform diameter or stepped, or may be provided with 4a cavity to house ignition composition. The `dielectric spacingV body may takecditferent forms. For example, If desired, part or all of the plug may be coated with a dielectric layer orlm. Other and more specific embodiments will be subsequently described.

` Having generally described the initiator of the invention', the novel structure thereof will be more particularly illustrated with reference to the accompanying drawing in which part sectional, part elevational views of various embodiments are shown and in which similar parts are designated by similar identifying symbols. i

InFig. l, an instantaneous electric blasting cap is shown having a metallicshell 1, a base charge 2, and a priming charge 3. Above the priming charge 3 is dis- A posed an ignition assembly comprising a semiconductive ignition plug 4 The plug 4 is stepped to form a lower 4portion of'lesser diameter. About the lower portion of the plug4 is disposed a cardboard spacing sleeve 5 which contacts the wall of the' shell V1. *'The spacing sleeve 5 maintains the plug 4 .inconcentric relationship with the shell 1 and provides a gapV 6 between the upper portion of the plug 4 and the shell 1. Bared terminal portions of otherwise insulated leadV wires 7`extend through the plugy 4 and are joined at their ends beneath the plug 4 by a bridge wire 8. An ignition composition 9 is disposed .about ,the bridge wire 8 inthe cavity formed by the portion of the spacing sleeve extending below the plug 4. An asphaltic sealing layer 10 is disposed above the plug 4 and a layer of sulfurL 11 is disposed above the asphalt.

.'The sulfur layer 11 is maintained in place by a bead 12.

Theinitiator shown in Fig. 2 is similar to that of Fig. 1. 'In Fig. 2, however, the semiconductive ignition plug is cylindrical and the dielectric spacing sleeve 21 has a thickness equal to width of the gap 6 between the plug V2() and the shell 1 above the sleeve 21.

In Fig. 3, is shown a cap similar in all respect to that of Fig. l, except for the provision of the cavity 31 in the stepped semiconductive ignition plug 30.

f The initiator of Fig. 4 is similar to that shown in Fig. 2 except for provision of the cavity 41 in the cylindrical ysemiconductive ignition plug 42.

In the cap of Fig. 5, a conventional shell 1, base charge 2, priming charge 3,y loose ignition composition 9, lead wires 7 and bridge wire 8 are employed. However, the jdielectric'spacing sleeve 51 does Ynot extend below theI -film 62 above the sleeve 61.

lower surface of the cylindrical semiconductive ignition plug 50. 'I'he lower surface of the plug S0 is coated with a dielectric film 52 of nitrocellulose lacquer to prevent static discharge from the base of the plug 50 to the shell 1 at the interface of the ignition composition 9`and the plug. A rubber sealingrplug 53 is crimped in the mouth of the shell 1 to complete the device.

The initiator of Fig. 6 is similar to that of Fig. l, ex cept for provision of a thin film 62 of rubber disposed in the gap between the upper portion of the semiconductive ignition plug 6G above the cardboard spacing sleeve 61. A rubber sealing plug 53 is crimped in the mouth of the shell 1 to complete the cap as in Fig. 5.

In Fig. 7 is shown a static resistant delay electric blasting cap in accordance with the invention. A conventional shell 1, base charge 2, priming charge 3, lead wires 7, and bridge wire 8 are employed. A delay fuse element 73 is disposed above the priming charge 3 and consists of a-lead tube 74 lilled with a delay train 75 composed from the wall by the thickness of the sheath V76. TheV plug is crimped in place to form a waterproof closure.

The delay electric blasting cap of Fig. 8 again employs l a conventional shellV 1, base charge 2,.priming charge 3, lead Wires 7, bridge wire 8, and delay fuse element 73. A matchhead ignition assembly 82 is employed to ignite the fuse and consists of a bead of ignition composition 9 formed aroundthe bridge wire 8. A dielectric shell liner 81 of cardboard is disposed in the shell 1 and extends` above the base of a stepped semiconductive igni- Vtion plug 80. 'The shoulder formed by the V,upper portion'of the plug 80 rests on the top of the liner 81 and the lowerportion of the plugv iits snugly into the liner 81. The upper portion of the plug 80 is thus spaced from the shell wall at a distance less than that between either lead wire and shell. -A rubber sealing plug`53 is crimped in the mouth of the shell 1 to complete the cap.

In all lof the'initiators illustrated in Figs. l8, static differential applied between lead wire and shell will discharge tothe grounded shell outside the locus of the ignition composition and therefore accidental ring from this type ofdischargeis prevented. In the caps of Figs. l, 3," and, 8, the distance between the semiconductive body and shell is "shortest above the sleeve or liner and discharge will, therefore, occur at this point. In the Adevices of Figs. 2 and 4, the spacingsleeves are of the same thickness as the gap 6, but the sleeves extend below the semiconductive Vplugs,'making the resistance of the path from the plug to the shell greater below or through the sleeve than above the sleeve. In Fig. 5; the spacing sleeve terminates at the base of the plug. However, the coating of the lower surface of the plug with a dielectric `film results again in less resistance todischarge .above the sleeve. In Fig. 6, the gap between the upper portion of the sleeve and shell is filled with dielectric material. However, since the distance to shell wall is still less above the sleeve, discharge takes-place through the In Fig. 7, a similar thin film is provided about the Ventire plug 70. Again, however, total resistance from the core471 to shell is less above the sleeve 72 and discharge takes place, along this The initiators of the invention are initially ladvanta* geous since they are easier to manufacture and are structurally stronger` than the. initiators of U. 82,658,451 Aabove referred to in that the semiconductive body and the ignition plug v'area singlebunit, `and in the preferred embodiments shown in Figs. 1-5 and 8, are aisingle body `of luniform composition. Moreover, the `increased amount of semiconductor thus economically available for contact -with the lead wires has been found to give great- `erprotection against static tiring of caps having a broken bridge wire. Even though the possibility of tiring can be reduced to practically nil by employing the semiconductive body and high heat capacity bridge wire of the parent application,it has been established in planned tests that where the bridge wire is loosened from the lead wire at -one end or is broken 'and atiny particle of static sensitive ignition composition is placed in the resulting gap, the cap of the parent Iapplication can be red with static `potential less than that which would otherwise tire the -cap by heating thebridge. Although such circumstances are remote, theA increased protection obtained by the present invention is desirable.

The increased protection readily `obtained `by employing semiconductive ignition plugs in accordance with the invention wasdemonstrated by preparing special experimental devices in which the `unbridged terminals of the lead wires were brought .to within .001 inch of each other. Highly sensitive lead styphnate `was placed between the terminals. Semiconductive bodies having the indicated composition were formed about `the lead wires. The static charge was applied across the `gap by a 750 mmf. condenser.

In the foregoing table the letter S represents "shot" and the letter `F represents failed It Will be noted that Va decreasingvamount of shots was obtained ineach case yas the height of the semiconductive plug was increased.

The static "resistance of the caps of the invention to a charge applied between shunted lead wires and shell is illustrated Table 2. Seismic caps were employed similar to that shown `in Fig. l. The semiconductive ignition `plugs were 4formed fromthe indicated compositions and were spaced from the shell -above the cardboard sleeve by aidistance of 0.015 inchand were`0.5 inch in height. The sleeve was .025 inch thick. The ignition composition was a conventional mixture of milled diazodnitrophenol-potassium 'chlorate employed in commercial seismic caps.

TABLE 2 35/65- 36/64 V 1t Wax/Aluminum Mix Wax/Aluminum Mix Number Tested Number Fired Number Tested Fired The semiconductive body acts as a nonconductor `for the :voltage employed to 'tire an electric initiator but is `a good conductor for a high voltage static discharge. Therefore, while the semiconductive body has no effect on the normal operation of the cap, it does constitute `a shunt for high voltage discharges between the lead wires within the shell when the static voltage is applied to both lead wires. This eifect is of particular benefit with a cap in which the bridge wire has been accidentally broken during manufacture. As indicated, formation of the ignition plug itself from the semiconductive material allows increased contact with the lead lwires without necessitating an increase in cap length. When the static voltage is applied across either of the wires and the shell, the charge is conducted from the lead wire through the semiconductive mass and arcs across the gap between the semiconductive body and the shell wall. Since the breakdown voltage of the semiconductive body `must be fairly low in order to `give the desirable protection against static discharge, the cap as a whole would have undesirably low voltage breakdown characteristics if the body were to contact the shell wall as is the case in several antistatic designs employed in prior art initiators. Because of the fact that the ignition plug is `spaced from the shell wall, the voltage breakdown value of the capi is `greatly increased and offers adequate protection against the type of stray current which can reasonably be expected to be found in blasting operations. For best results it is preferred to space the semiconductive plug from the shell wall by a distance of between 0.005 and 0.050 inch.

When the strength of the static charge results in a passage of the current through the bridge wire, despite the shunt formed by the semiconductive body, the use of a high heat capacity Wire in accordance with application Serial No. 340,821 impedes the generation of suicient heat in the bridge to ignite the ignition composition. The heat capacity of a bridge wire can be increased by increasing the length of the diameter. The dimensions of an electric initiator and manufacturing considerations place a limitation on the permissible length of a bridge wire,

and it is consequently preferred to increase heat capacity by increasing the diameter. An increase in diameter will give .the desired increase in protection against static electricity but for a given material the increase in diameter will also -result in a corresponding increase of the minimum tiring current of the cap. While this fact has no bearing upon the reduction of static susceptibility of the initiator, it is desired to maintain the minimum tiring current of-an electric initiator at a low level in the neighborhood of 0.5 to 1 amp, Therefore, if, in accordance with the present invention, the diameter of the wire is to be increased, it is desirable to employ a metal or alloy in the bridge wire which is characterized by a high specific resistance and a high specic heat. It has been found that with most of the ordinary materials used by the art, the minimum tiring current of the initiator is undesirably raised if the diameter of the bridge wire exceeds about 0.0015 inch, which is the usual diameter employed by the art. It has been found that greatly increased static resistance is obtained if a diameter of 0.0020 inch is employed. It is preferred to employ a bridge wire diameter of about 0.0025 inch and in order to avoid any substantial increase in `minimum tiring current, it is preferred to employ a bridge wire made from a nickel-chromium alloy.

Although it is usually preferred to employ afconductive metal powder as the conductive component of the semiconductive ignition plug, especially aluminum powder, other particulate conductive materials, such as conductive carbon may be employed. The nonconductive component which acts as a binder and a carrier for the conductive particles is preferably wax. The wax ernployed will desirably have a melting point of F. or above in order to prevent substantial cold flow of the wax subsequent 'to manufacture. The melting point of `the wax, oipany other operable carrier, will,iof course, depend the like. '-For most purposes cardboard is prefe'riiedfdue largely upon practical Y considerations. For instance, if to` its' ease of fabrication, economy, and excellent dithe Waterproofing material Aerriployed is an asphaltic corrielectric qualities. "l position which is poured into the cap in molten form, a If it is, desired to employ the structureKV of Fig.v 5`,fan'y wax willi-be employed which has a melting point subsuitablefmaterial/rnay be employedas the dielectric ylilm 'stantially higher than the temperature of the iilling com- 52. Lacquer Vis'preferred due toeaseofapplictioii. Genposition as it is poured into the shell.l On theother hand, erally, however, .it isfpreferred to employ structure in if a rubber or resin sealing plug is employed and is placed Vwhich the spacing body V extends below thebase ofVV the in position in coldV form, the only consideration then is to semiconductive plug unless the plug is stepped as in Fig. 3. use a-Wax which will not c old ow. If a molten resin is As will be apparent Yto those in lthe initiatorairt, the employed, the sameiconsiderations then pertain as in the conventional componentsof-the initiators may be replaced case of a molten asphaltic composition. Preferred waxes by equivalents. The shells may be of any conductive include such natural waxes as candelilla, montan, and metal such as aluminum, brass, copper, ferrous' metals and .camaubavand Synthetic waxes. Various alloys. The Alead wiresv inay Vbe madel ofiany j Instead of wax,` however, rubber or rubber-like mal5 of the Vconventionl*materials such as copperand iron, and terials, resinous materials, vsulfur and equivalent mamay be tinned if desired; The `lead Vwires may beinterials may be employed. Since it is desirable that the sulated with any desired material suchas cotton'servinigs, lbody ofsemiconductive material be molded about the rubber or various plastics. ,I i lead wires, readily moldable nonconductive materials are The-base charges may be formed from any secondary preferred Y detonative explosive ,such as pentaerythritol.tetranitrate, Itis preferred that the annulus between the semi- Cyclonite, tetryl, trinitrotoluene, and the like, and may conductive plug and the shell above the dielectric spacing be cast or pelleted as well asr pressed when the-.nature body be substantially unfilled except by air. If the shell of the explosiveV permitsl 7 The priming charge. may be issealed by a molten material which hardens in situ, some omitted if the base charge is Ycapable of initiation by the 'of this material may ow into the annulus. It has been 25 action of the ignition composition. When a priming found in practice, however, thatl this seldom occurs, charge is employed as is preferred, any primary explosive probably due to the fact that the air is not `readily disor mixture may be used Such aS, diazodinitrophenol, diazoplaced. Flow of molten sealing material into the annulus dinitrophenol-potassium chlorate, lead azide, lead stycan be promoted, if desired, by preheating the shell. Imphnate, and mercury fulminate. Any of the known igniproved static resistance is still obtained, however, even tion compositions may also be employed such as finely 'when the annulus is filled with an insulatory material, divided diazodinitrophenol-chlorate mixture, fulminates,

since the static will still discharge through theV thin layer. leador tin-selenium mixtures, etc. i i In Figs. 6 and 7, a thin dielectric layer of rubber has been As indicated in the drawing',l the ignition assemblies employed about the plug where discharge occurs, but other may employ cavity ignition plugs, matchheads orrloose dielectric, insulatory materials may be employed such as 3" l ignition chargesV in accordance withY conventional strucsulfur,A asphalt, synthetic resins, and the like. Y tures employed lin the blasting initiator art. The lshell The structures of Figs. l6 and 7 are advantageous in that may be sealed by more Vthan one Vsealing layer or, as protection is automatically present against inadvertent indicated, a single seal may be employed such as a contactbetween core and shell if the plug is not properly rubber sealing plug or a castor molded resinA plug, and centered in the shell. The structures `ot Figs. l-4 and 8 40 the shell suitably crimped. i f c an be modified to aord similar protection by employing VVSince, .as indicated, the initiators within the Vscope of a narrow dielectricrcollar or band around the upper porthe invention can' be altered in manyV rrespects Without tion of the semiconductive ignition plug. Alternatively, Changing their mode `of operation, itis intended that small dielectric beads or spacing members may be emr, the invention'be limited only by the scope of the appended vployed. `In a further embodiment, a dielectric spacing "J claims.

disc may be mounted on top of the ignition plug. WhatI claim and desire to protect by Letters Patent is.: Further, the dielectric `sleeve disposed between the upper 1 A Static resistant electric initiator having in conportion Nof Athe iginition plug and shell in Fig. 6 may be blnation a metallic shell, a pair of lead wires, an ignition Vperforated to facilitate discharge while still insuring 50 plug 0f semiconductive material'idisposed about bared proper spacing. portions of the lead Wires'within-the shell, a bridge wire The amount or' a particular conductive ller employed oining the terminals of the lead WIeS belOW theV IOWCI lin the semiconductive body will depend on the conduc- Surface of the pli-ig, an ignition compositiondifeotly below tivity of the particular material and on the degree of Said Ping andV diSP-oiSed about the bl'idgeWiIe, a Spacing static protection desired. The method of forming the 5,. body of dielectric material Surrounding a lower Portion semiconductive bodyabout the lead wiresv will also diei 0f the plug between the plug and shell wall, the portion tate the optimum .quantity'of particulate conductive'filler of the Ping aboVe the Spacing body approaching the Shell employed. In the preferred composition of wax and alu- Wall but being Separated therefrom by a distance Substan- `minum, it has been found that best results are obtainable tially less than that between either lead wireand the shell Ywhen between and 70% of particulate aluminum is 60 in the locnS of the ignition coInPoSition, and the total f :employed The upper limit 0f 70% is primarily dictated resistance between the shell wall and plug being substan-V by the fact that itis desired to employ a pourable mixture p tially less above thespacing body than at any'other point 'for molding. y The use of more than 70% of aluminum on the plug.

ppwder'usually results in a mixture which is undesirably 2. An electric initiator in accordance with claim 1 in viscous. It has been fOllnd that adequate Protection which the semiconductive material compris-eisV particulate against Staticdi'scllafge from' the lead Wife tothe Shell conductive material in admixture with a nonconductive through the ignition composition can be obtained with binder Y Y.

about 60% o* Partlculate alummum Lesser amounts of 3.` An electric 'initiator in .accordance with claim 1 in l d 'thve b re lt.

aluminum can be emp oye W1 ry eneclal su s which the semiconductive material comprises'particulate From all considerations a 35/ 65 mixture of wax/aluminum has been found to give excellent results and is 10 conductive metal powderin admixture with a nonconducpreferred. tive binder. l Y,

As indicated in the drawing, the dielectric spacing body An electric initiator in accordance hclam 1 in may have Various configurations and may be made of which the semiconductive material comprisesparticulate `many different solid dielectric materials. Such materials aluminum and wax.

finclude cardboard, paper, berboard, rubber, plastic, and 75 5.l An electric initiator in accordance with claim 1 in 9 which the semiconductive material comprises from 60 to 70% of particulate aluminum and 40 to 30% of wax.

6. A static resistant electric initiator having in combination a metallic shell, a pair of lead wires, an ignition plug of semiconductive material disposed about bared portions of the lead wires within the shell, a bridge wire joining the terminals of the lead wires below the lower surface of the plug, an ignition composition directly below said plug and disposed about the bridge wire, a sleeve of dielectric material surrounding a lower portion of the plug between the plug and shell wall and extending below the plug to form a housing for ignition composition, the portion of the plug above the sleeve `approaching the shell wall but being separated therefrom by a distance substantially less than that between either lead wire and the shell in the locus of the ignition composition, and the total resistance between the shell wall and plug being substantially less above the sleeve than at any other point on the plug.

7. An electric initiator in accordance with claim 6 in which the semiconductive material comprises particulate conductive material in admixture with a nonconductive binder.

8. An electric initiator in accordance with claim 6 in which the semiconductive material comprises particulate conductive metal powder in admixture with a nonconductive binder.

9, An electric initiator in accordance with claim 6 in which the semiconductive material comprises particulate aluminum and wax.

10. An electric initiator in accordance with claim 6 in which the semiconductive material comprises from 60 to 70% of particulate aluminum and 40 to 30% of wax.

11. A static resistant electric initiator having in combination, a metallic shell, a pair of lead wires, an ignition plug of semiconductive material disposed about bared portions of 'the lead wires within the shell, said plug having a lower portion of reduced diameter, a bridge wire joining the terminals of the lead wires below the lower surface of the plug, an ignition composition directly belowv said plug and disposed about the bridge wire, and a sleeve of dielectric material of uniform wall thickness surrounding the portion of the plug having a reduced diameter and said sleeve having a portion of reduced wall thickness extending over the upper portion of the plug to space said upper portion from the shell wall, said upper portion of the plug being separated from the shell wall by a distance substantially less than that between either lead wire and the shell in the locus of the ignition composition.

12. An electric initiator in accordance with claim ll in which the semiconductive material comprises particulate conductive material in admixture with a nonconductive binder.

13. An electric initiator in accordance with claim 1l in which the semiconductive material comprises particulate conductive metal powder in admixture with a nonconductive binder.

14. An electric initiator in accordance with claim 11 in which the semiconductive material comprises particulate aluminum and waX.

15. An electric initiator in accordance with claim l1 in which the semiconductive material comprises from 60 to 70% of particulate aluminum and 40 to 30% of wax.

16. A static resistant electric initiator having in combination a metallic shell, a pair of lead wires, an ignition plug of semiconductive material disposed about bared portions of the lead wires within the shell and coated on its undersurface with a layer of dielectric material, a bridge wire joining the terminals of the lead wires below the lower surface of the plug, an ignition composition directly below the said coated plug and disposed about the bridge wire, a spacing body of dielectric material surrounding a lower portion of the plug between the plug and shell wall, the portion of the plug above the spacing body approaching the shell wall but being separated therefrom by a distance substantially less than that between either lead wire and the shell in the locus of the ignition composition, and the total resistance between the shell wall and plug being substantially less above the spacing body than at any other point on the plug.

l7. A static resistant electric initiator having in combination a metallic shell, a pair of lead wires, an ignition plug of semiconductive material disposed about bared portions of the lead wires within the shell and coated on its undersurface with a layer of dielectric material, a bridge wire joining the terminals of the lead wires below the lower surface of the plug, an ignition composition directly below the said coated plug and disposed about the bridge wire, a sleeve of dielectric material surrounding a lower portion of the plug between the plug and shell wall and extending below the plug to form a housing for ignition composition, the portion of the plug above the sleeve approaching the shell wall but being separated therefrom by a distance substantially less than that between either lead wire and the shell in the locus of the ignition composition, and the total resistance between the shell wall and plug being substantially less above the sleeve than at any other point on the plug.

18. A static resistant electric initiator having in combination a metallic shell, a pair of lead wires, an ignition plug of semiconductive material disposed about bared portions of the lead wires within the shell and coated on its undersurface with a layer of dielectric material,

said plug having a lower portion of reduced diameter, a bridge wire joining the terminals of the lead wires below the lower surface of the plug, an ignition composition directly below the said coated plugand disposed about the bridge wire, and a sleeve of dielectric material of uniform wall thickness surrounding the portionof the plug `having a reduced diameter and said sleeve having a portien of reduced wall thickness extending over the upper portion of the plug to space said upper portion from the shellvwall, said upper portion of the plug being separated from'the shell wall by a distance substantially less than that between either lead wire and the shell in the locus of the ignition composition.

References Cited in the le of this patent UNITED STATES PATENTS 2,240,438 Durant n Apr. 29, 1941 2,408,125 Rolfes Sept. 24, 1946 2,658,451 Horne Nov. 10, 1953 2,685,835 Noddin et al Aug. 10, 1954 

1. A STATIC RESISTANT ELECTRIC INITATOR HAVING IN COMPBINATION A METALLIC SHELL, A PAIR OF LEAD WIRES, AN IGNITION PLUG OF SEMICONDUCTIVE MATERIAL DISPOSED ABOUT BARED PORTIONS OF THE LEAD WIRES WITHIN THE SHELL, A BRIDGE WIRE JOINING THE TERMINALS OF THE LEAD WIRES BELOW THE LOWER SURFACE OF THE PLUG, AN IGNITION COMPOSITION DIRECTLY BELOW SAID PLUG AND DISPOSED ABOUT THE BRIDGE WIRE, A SPACING BODY OF DIELECTRIC MATERIAL SURROUNDING A LOWER PORTION OF THE PLUG BETWEEN THE PLUG AND SHELL WALL, THE PORTION OF THE PLUG ABOVE THE SPACING BODY APPROACHING THE SHELL WALL BUT BEING SEPARATED THEREFROM BY A DISTANCE SUBSTANTIALLY LESS THAN THAT BETWEEN EITHER LEAD WIRE AND THE SHELL IN THE LOCUS OF THE IGNITION COMPOSITION, AND THE TOTAL RESISTANCE BETWEEN THE SHELL WALL AND PLUG BEING SUBSTANTIALLY LESS ABOVE THE SPACING BODY THAN AT ANY OTHER POINT ON THE PLUG. 